1. Field of the Invention
The present invention relates a polishing apparatus that polishes an object by causing a relative movement between a polishing body and the polishing object while causing the polishing object to contact the polishing body, and specifically concerns a polishing apparatus that is capable of detecting an endpoint of polishing of the polishing object.
The present invention also relates to a film thickness inspection method used in semiconductor processes, and specifically relates to a film thickness inspection method that is suitable for use in film thickness measurement and control in polishing processes.
2. Discussion of the Related Art
In recent years, as a result of an increased degree of integration, semiconductor integrated circuits have utilized both increasingly narrow line widths formed by using a lithography, or similar process, and an increase in the number of laminated layers. As the line-widths have narrowed, the light source wavelengths used in photolithography have become shorter, resulting in a larger numerical aperture ("NA"). Furthermore, the surface shapes of the semiconductor devices are no longer always flat, creating additional problems and additional concerns.
The presence of step differences on the surfaces of semiconductor devices leads to step breaks in wiring and local increases in resistance, thus causing wiring breaks and drops in current capacity. These problems are further compounded where layers are laminated on top of previously patterned layers, projections, and indentations. The patterns in the lower layers are reflected in the surface shapes of the overlying layers, so that steps are created in the surfaces of the upper layers. When wiring layers are laminated on top of layers with such steps, breaks in the wiring layer or local increases in resistance may occur. Where insulating layers are formed on top of layers that have steps, the over-voltage performance of such insulating layers deteriorates and voltage leakage may occur. Moreover, in cases where exposure by photolithography is attempted on layers that have steps on their surfaces, the optical focusing system of the exposure apparatus cannot be focused in the step areas. The occurrence of such defects caused by the steps becomes more conspicuous as the number of layers that are laminated increases.
Accordingly, one proposal has been to remove surface steps by applying polishing processes to the surfaces of the upper layers where further layers are laminated on top of patterned layers. A polishing apparatus of the type shown in FIGS. 16A and 16B has been proposed to remove the surface steps. The apparatus uses a technique known as "chemical mechanical polishing" or "chemical mechanical planarization" (hereafter referred to as "CMP"). This technique is based on polishing of silicon wafers technology. Specifically, in this apparatus, a polishing cloth 1602 (including one or two layers) is pasted to the surface of a rotationally driven base plate 1601, which has a high rigidity, while a wafer 1604 is held in a holder 1603. The wafer 1604 then contacts the surface of the polishing cloth 1602. While the base plate 1601 is rotationally driven, the holder 1603 rotates in the same direction as the base plate 1601 while a load is applied to the holder 1603 from above. A polishing agent 1606, such as acids or alkalies, is then discharged onto the polishing cloth 1602 from a polishing agent discharge port 1605 so that the polishing agent 1606 is applied to the polished surface and the wafer 1604 is polished to a flat surface.
Various techniques are used by various processes during the manufacture of semiconductor devices, with the final state of the flattening polishing varying according to the process involved. For example, in wafer 1604, as shown in FIGS. 17A-D, shallow grooves 1705 used for element separation (shallow trench isolation) are formed in a substrate 1704 and the grooves 1705 are mainly filled with an oxide film filler material 1706, as shown in FIG. 17B. The filler material 1706 is removed by polishing, and the flattening polishing is completed when the undersurface 1707 is exposed in areas other than the grooves 1705, as shown in FIG. 17C.
In the so-called "Damascene" process, as shown in FIG. 18, the grooves 1805, which serve as wiring areas, are formed by etching an insulating film 1804 on the surface of a substrate 1704, as shown in FIG. 18A. A metal wiring material 1806, such as aluminum or copper, is embedded in the grooves 1805, as shown in FIG. 18B. The metal wiring material 1806 is then removed by polishing, and the flattening polishing is completed when the insulating film 1804 in areas other than the wiring areas of the grooves 1805 is exposed, as shown in FIG. 18C. Although it is not shown in the figures, the polishing apparatus is also used in the flattening polishing processes that are performed after the inter-wiring connections (called "through-holes" or "via holes") are filled with a conductive material, such as polysilicon, tungsten, aluminum, or a similar material. The flattening polishing process is completed when the insulating film is exposed.
Conventionally, endpoint detection has been accomplished by a system in which the torque of the motor (not shown in the figures) driving the base plate 1601 is monitored. Specifically, as polishing of the waver 1604 progresses, the characteristics of the polished surface changes, so that the torque required in order to drive the base plate 1601 also changes. For example, if the current supplied to the motor driving the base plate 1601 is monitored at a fixed voltage, the endpoint of the flattening polishing process can be detected from the fluctuation of the current.
The change in torque will be described with reference to FIGS. 17A-17D and 20. For example, when the filler material 1706 is polished so that the surface is flattened, as shown in FIGS. 17A-17D, the torque becomes approximately constant as indicated by portion P of the characteristic curve, as shown in FIG. 20, so that fluctuation is reduced. As the surface is further polished, the filler material 1706 is removed from areas other than the grooves 1705 so that polishing is completed. The undersurface 1707 is thus exposed resulting in-changed surface conditions. As a result, the torque becomes approximately constant at a lower torque level as indicated by portion Q of the characteristic curve, as shown in FIG. 20. The difference between the torque levels associated with the different materials makes it is possible to detect the endpoint of the polishing process.
Generally, the occupation rate of the grooves 1705 (i.e., the proportion of the area occupied by the grooves 1705 at the surface of the wafer 1604) is small. The filler material 1706 and undersurface 1707, in areas other than the grooves 1705, have different coefficients of kinetic friction. Thus, the amount of fluctuation in the torque is large, so that the endpoint of the polishing process can be detected relatively easily. However, the proportion of the area occupied by the grooves 1705 is not always small; furthermore, the filler material 1706 and the undersurface 1707 do not always have different coefficients of kinetic friction. If the occupation rate is large, or the filler material 1706 and the undersurface 1707 have approximately the same coefficient of kinetic friction, the amount of fluctuation in the torque is small even when the polishing process is completed. Therefore, precise endpoint detection is diminished and depending on the conditions the detection of the endpoint, detection of completion of the flattening polishing process becomes difficult. A similar problem occurs in the flattening process shown in FIGS. 18A-18C.
Additionally, there are flattening processes wherein the surface steps in the outermost surface layers of the substrates are removed by CMP, and it is necessary to measure the film thickness of the outermost surface layers in order to determine whether the outermost surface layers have been polished to the desired film thickness. This process is employed because there is no change in the surface shape or surface characteristics, and hence there is no corresponding change in the motor torque as the materials change due the polishing process when the process is completed. Therefore, it is virtually impossible to detect the endpoint using the torque detection method. Such a process is shown in FIGS. 19A and B.
In this process, wiring 1904 is formed on the surface of a substrate 1704, as shown in FIG. 19A, and the wiring 1904 is covered by an inter-layer insulating film 1905 as shown in FIG. 19B. The surface of the inter-layer insulating film 1905 is then flattened by polishing and the flattening polishing is completed when the inter-layer insulating film 1905 thickness over the wiring 1904 reaches a pre-set value TO.
Another conventional method has been proposed for detecting the film thickness, wherein the film thickness is measured using light interference by illuminating the outermost surface layer with light and detecting the reflected light. Specifically, the endpoint is detected by forming slits in the base plate and polishing cloth, illuminating the polished surface of the wafer via the slits with a laser beam from a laser beam light source installed beneath the base plate, and detecting the reflected light with an interferometer.
Unfortunately, the light measuring interference method described above creates further complications. For instance, although the light interference detection method may solve the film thickness measurement problem, the same endpoint detection region should always be detected. However, the wafer 1604 and base plate are rotating, and thus it is difficult to detect the same endpoint detection region in all cases.
Furthermore, the substrates, wherein CMP process is employed, have circuit patterns formed on the underlying layers resulting in a non-uniform light reflectivity of the underlying layers. Accordingly, even if the outermost surface layer is illuminated with light in order to measure the film thickness, the distribution of the reflectivity of the underlying layers effects the results, so that the film thickness cannot be accurately measured.